Aromatic polyether sulfones are polymers whose monomeric self-repeating units have at least
(a) a sulfone group between two aromatic radicals and
(b) an ether linkage between two aromatic radicals, the two aromatic radicals between which the sulfone group is located being in each case linked to another aromatic radical via an ether linkage. The attachment of the aromatic radicals present in the monomeric unit to the corresponding substituents can in each case occur in the ortho, meta or para position. The aromatic radicals may optionally be further substituted, in which case, however, at least one hydrogen atom linked to the aromatic ring is present in each aromatic radical of the monomeric unit, which atom may be substituted by the sulfonic acid group by means of a sulfonation reaction.
Typical representatives of such polyether sulfones are polymers having the following self-repeating structural unit ##STR1## Such polymers are commercially available products.
Aromatic polyether sulfones are known polymers which are already in use in various areas of application, use being made of their good thermal, mechanical and chemical stability. Examples of areas of application are automotive spare parts, electrical connections and coatings.
Polyether sulfones have also already been utilized for the preparation of membranes used in materials separation processes, use being made of the resistance of the material, for example, to water, chlorine and a number of organic solvents. Membranes made from polyether sulfones are described, for example, in EP-A 0,121,911.
Membranes made of such materials are, for a number of applications which require a highly hydrophilic character, insufficiently hydrophilic; this drawback manifests itself, for example, by insufficiently rapid wetting by liquid aqueous systems.
Attempts were therefore made to modify polyether sulfones by introducing ionic or dissociable substituents. One such possibility is to introduce sulfonic acid groups which may be subsequently converted to salts. The substitution of the aromatic polyether sulfones takes place on the aromatic ring system and is frequently carried out using a sulfonating agent. Chlorosulfonic acid may be used for this purpose as the sulfonating agent. A corresponding process is described, for example, in U.S. Pat. No. 4,508,852. Using chlorosulfonic acid has the disadvantage of side reactions occurring. Thus the method of the U.S. Patent referred to above gives rise not only to sulfonated, but also to chlorosulfonated products. Furthermore, the reaction in organic solvents at temperatures of more than 40.degree. C. described therein leads partially to the formation of undesirable crosslinked polymers.
Attempts to sulfonate polyether sulfones using concentrated sulfuric acid indicated that this method was applicable only to those polymers whose monomeric units are substituted in a suitable manner. Thus it can be concluded from EP-A 0,008,894 that phenylene radicals which have always an ether linkage in the 1,4-position, may be sulfonated using concentrated sulfuric acid, but this does not hold for phenylene radicals which have a sulfonic group and an ether linkage together in para-position. The reason for this is presumably to be found in the strongly electron-attracting effects of the sulfonic group. It is true that EP-A 0,008,894 states that these less reactive phenylene groups referred to can be acted upon by chlorosulfonic acid or oleum, but the use of these sulfonating agents is discouraged since they are said to lead to a high degree of sulfonate and/or polymer degradation. A controllable sulfonation reaction using chlorosulfonic acid or oleum is said not to be possible. EP-A 0,112,724 makes the same assertion.
According to U.S. Pat. No. 3,709,841, sulfonation of polyether sulfones is successful even using chlorosulfonic acid or oleum if the reaction is carried out in an inert solvent. Suitable solvents are said to be chlorinated hydrocarbons. The disadvantage of using these solvents is, inter alia, their toxicity. Removal of the solvents used also creates problems.
Accordingly, in contrast to the known methods described, sulfonation using concentrated sulfuric acid containing sulfur trioxide (oleum, fuming sulfuric acid) as solvent would be desirable.